Three patients with a type of myoclonus produced by intention and somatosensory stimulation were studied with electrophysiologic techniques. Each jerk typically affected only a few contiguous muscles; agonist and antagonist muscles were activated simultaneously with a simple electromyographic (EMG) burst lasting 10 to 30 msec. Cranial nerve muscles were activated in an order indicating that the signal to produce the myoclonus traveled down the brainstem. In action-induced jerks a negative transient in the electroencephalogram (EEG) from the contralateral sensorimotor cortex consistently preceded the jerk with a fixed latency. In reflex-induced jerks this negative transient could be recognized as a component of the sensory evoked potential. The types of myoclonus are reviewed and it is argued that this type of myoclonus is mediated in cerebral cortex and that the negative transient represents a paroxysmal depolarization shift (PDS). The myoclonus may result from hyperactivity of a component of the long-latency stretch reflex.